Venous blood flow returns de-oxygenated blood from the distal extremities to the heart via two mechanisms. The first is the perfusion pressure resulting from the arterial blood flow through tissue to the venous circulation system. Where arterial pressure prior to perfusion may be 60 to 200 mm Hg, the resulting venous pressure is typically 10 to 40 mm Hg. The second mechanism is the calf muscle, which, when contracted, compresses the veins (tibial and peroneal) overlying the bone and, through a system of valves, directs blood flow toward the heart. This is the organized flow of blood through a normal, healthy person.
Venous valves, especially those in the upper leg, perform an important function. When a person rises from a seated to a standing position, arterial blood pressure increases instantaneously to insure adequate perfusion to the brain and other critical organs. In the legs and arms, the transit time of this increased arterial pressure is delayed, resulting in a temporary drop in venous pressure. Venous pressure drops as blood flow responds to body position change and gravity, thereby reducing the volume of blood available to the heart and possibly reducing the flow of oxygenated blood to the brain. In such a case, a person could become light headed, dizzy, or experience syncope. It is the function of valves in the iliac, femoral and, to a lesser degree, more distal vein valves to detect these drops in pressure and resulting change of direction of blood flow and to close to prevent blood from pooling in the legs to maintain blood volume in the heart and head. The valves reopen and the system returns to normal forward flow when the reflected arterial pressure again appears in the venous circulation. Compromised valves, however, would allow reverse blood flow and pooling in the lower legs resulting in swelling and ulcers of the leg. The absence of functioning venous valves can lead to chronic venous insufficiency.
Techniques for both repairing and replacing the valves exist, but are tedious and require invasive surgical procedures. Direct and indirect valvuoplasty procedures are used to repair damaged valves. Transposition and transplantation are used to replace an incompetent valve. Transposition involves moving a vein with an incompetent valve to a site with a competent valve. Transplantation replaces an incompetent valve with a harvested valve from another venous site.
Prosthetic valves can be transplanted into the venous system, but current devices are not successful enough to see widespread usage. One reason for this is the very high percentage of prosthetic valves reported with leaflet functional failures due to excessive protein deposit, cell growth and thickening, and thrombosis. These failures have been blamed primarily on improper sizing and tilted deployment of the prosthetic valve. In addition, a great number of the valve leaflets come into close proximity to or in contact with the adjacent vessel wall, conduit, or supporting frame of the valve. Such contact or proximity can cause regions of blood stasis or near stasis, and can result in increased thrombus formation. In addition, contact can result in disruption of endothelial or other tissue at the contact point, further increasing the likelihood of thrombosis or increased tissue deposits or healing response. Further, such contact or proximity can result in valve leaflet(s) being stuck to the vessel wall, or otherwise unable to move properly, thereby rendering the valve less functional or completely non-functional.